The nice thing about urban preppers is that's the place to go in a doomsday. They have a ready supply of water, canned goods, and ammo.

I am sure some preppers will in fact be ready, guns loaded, and should not be approached. These are also likely to be the ones avoiding urban areas anyway. But the ones I know (fortunately none of whom read HN AFAIK) well enough to know their prep...are unlikely to actually have their guns readily available.